The invention relates to a device for sweeping a laser beam focus.
Power lasers make it possible to focus high luminous energy in a small volume focus and can be applied in physics to the study of particles, plasma confinement, machining, or surgical organic tissue section. Most of these applications involve moving the focus so that it sweeps across a trajectory, which is difficult to achieve in a simple way. Since the laser itself is voluminous, it is preferable to leave it immobile and to move the beam only. This is possible by passing it along an optic fibre, but not in all cases, and in particular not outside frequencies beyond the visible spectrum, whereas infrared lasers are greatly appreciated because of their power. Another means of displacing a beam consists of passing it by using a turning mirror, but movement of the focus in all directions can only be obtained by using a succession of mirrors turning around different axes, which poses considerable problems for coupling the position of the mirrors, since a rotation of one mirror necessitates displacing all the downstream mirrors so that the beam continues to pass between them. Furthermore, a large part of the energy is finally lost.
This is the reason why it is thus proposed to sweep a trajectory for a laser beam focus without moving the beam, but by modifying its focal distance. This is achieved by means of a variable curvature mirror, whose curvature variation is associated with a variation in the focusing of light beams; the flexibility of the membrane allows significant deformations and thus makes long sweepings possible. It should be noted that such mirrors are already known to those skilled in the art in telescopes or other optical devices, but herein the variation of focal length was used in particular to adjust the apparatus and to provide a clear image of a natural radiation (see French patent 2 662 512). The deformation is often controlled by networks of piezoelectric sensors whose displacements determine a two-dimensional profile of the membrane, but such actuators have only a small displacement range and their simultaneous control of different displacements is costly. They are poorly adapted for imposing regular curvatures (rather than local displacements), especially if the membrane is submitted to big overall displacements. An example of such prior art is given in U.S. Pat. No. 4,934,803-A.
To resume, the invention relates, in its most general form, to a device for sweeping a laser beam focus, comprising a flexible and stretched membrane onto which the beam is projected and reflected upstream from the focus, adjustable means for varying the curvature of the membrane and means for cooling the membrane. Greater possibilities for adjusting the focusing can be obtained if the means for varying the curvature are double and comprise a means for applying pressure distributed over the membrane and a means for applying pressure concentrated on the membrane. The means for distributed pressure application is a means for varying pressure from a closed chamber defined by the membrane, and the means for concentrated pressure application is an electrical circuit with a direct voltage generator between the membrane and an electrode adjacent to a central part of the membrane.
The means for cooling the membrane are essential for evacuating the incident heat from the laser. They can comprise means for blowing gas across a face of the membrane, and these means can consist of a circuit for renewal of the gaseous contents of the closed chamber.
Finally, devices for inclination of the membrane can allow adjustment of the orientation of the mirror and therefore the position of the focus of the beam in various directions.